Effect of development on [Ca ²⁺] _i transients to ATP in petrosal ganglion neurons: A pharmacological approach using optical recording

ATP, acting through P2X ₂/P2X ₃ receptor-channel complexes, plays an important role in carotid body chemoexcitation in response to natural stimuli in the rat. Since the channels are permeable to calcium, P2X activation by ATP should induce changes in intracellular calcium ([Ca ²⁺] _i). Here, we describe a novel ex vivo approach using fluorescence [Ca ²⁺] _i imaging that allows screening of retrogradely labeled chemoafferent neurons in the petrosal ganglion of the rat. ATP-induced [Ca ²⁺] _i responses were characterized at postnatal days (P) 5-8 and P19-25. While all labeled cells showed a brisk increase in [Ca ²⁺] _i in response to depolarization by high KCl (60 mM), only a subpopulation exhibited [Ca ²⁺] _i responses to ATP. ATP (250 -1,000 μM) elicited one of three temporal response patterns: fast (R1), slow (R2), and intermediate (R3). At P5-8, R2 predominated and its magnitude was attenuated 44% by the P2X ₁ antagonist, NF449 (10 μM), and 95% by the P2X ₁/P2X ₃/P2X _2/3 antagonist, TNP-ATP (10 μM). At P19-25, R1 and R3 predominated and their magnitudes were attenuated 15% by NF449, 66% by TNP-ATP, and 100% by suramin (100 μM), a nonspecific P2 purinergic receptor antagonist. P2X ₁ and P2X ₂ protein levels in the petrosal ganglion decreased with development, while P2X ₃ protein levels did not change significantly. We conclude that the profile of ATP-induced P2X-mediated [Ca ²⁺] _i responses changes in the postnatal period, corresponding with changes in receptor isoform expression. We speculate that these changes may participate in the postnatal maturation of chemosensitivity.